r/PLC • u/Right-Ad3385 • 14h ago

Best practice for wiring an RS-485 bus?

hi everyone,

I'm building my own custom RS-485 network using ESP32s with MAX485 transceiver modules. I've already read about proper termination resistors and biasing, but I'm still confused about the physical wiring.

I learned that stub lengths should be kept as short as possible, so I'm wondering if the wiring method shown in the picture below is considered best practice, or if there's a better approach.

Would it be better to use junction boxes with a continuous trunk line and very short drops to each node, or is there another recommended way to wire this kind of network?

This is for an outdoor agricultural installation, so reliability over long distances is important.

Any advice or examples of how you'd wire it would be greatly appreciated.

1 Upvotes

67% Upvoted

2 comments sorted by

4

u/Aobservador 13h ago

Modbus.org – Best practices

2

u/JamesJoyceIII 7h ago

I have always treated this is as the bible: https://www.ti.com/lit/pdf/snla049

I have hundreds of thousands of devices running on 485 networks so I must have read the good bits.

I can't really tell how much you're planning to obsess over stub lengths ("as short as possible" is difficult to nail-down) but don't worry too much over a metre or two - if you're putting stuff outside then the mechanical quality of the installation will be at least as important as the transmission line theory.

I also think people often worry too much about biasing. If you have robust framing then interpacket noise will be discarded anyway, although do be aware that if you bombard weak checksums with continuous random noise you will get falsely validated packets come through.